Permanent magnets that are used for electrostatic developing magnet rolls, electric apparatus motors, and actuators were limited mainly to hard ferrite magnets; but, they suffer from problems such as low temperature demagnetizing characteristics at low temperature below iHc, and due to the nature of ceramic material, they have low mechanical strength, which is likely to result in cracking and chipping, and it is difficult to obtain a complex shape.
Today, miniaturization of household electric appliances and OA equipments has advanced, and magnet material used must be miniaturized and lightened. That is to say, in order to conserve energy, less weight of an automobile to gain better mileage is strongly sought, and the demand for miniaturization and reduction in the weight of automobile electric apparatuses.
Therefore, for the purpose of maximizing the performance to weight ratio of magnetic material, designing efforts to achieve that goal are in progress. For example, Br of 5.about.7 kG is considered most appropriate as magnet material in the present motor design. That is to say, in the present motor design, when Br exceeds 8 kG the cross sectional area of iron plates or rotor and stator which will become a magnetic path need to be increased, which instead will result in an increase in weight. Also, due to miniaturization of a magnet roll and a speaker, a magnet with high Br is desired, but the usual hard ferrite magnet cannot reach the residual magnet flux density (Br) in excess of 5 kG.
For example, although a Nd--Fe--B type resin bonded magnet satisfies the necessary magnetic characteristics, it contains 10-15 at % of Nd, which requires many processes and a large scale production facility in separation, purification and reduction of the metal. It is not only very expensive in comparison to hard ferrite magnet, but also it requires nearly 20 kOe of magnetizing magnetic field to magnetize 90% of the magnet, so that it is impossible to perform the complex multipolar magnetization necessary for a magnet for a magnet roll or other application such as stepping motors. At present, no one has discovered a magnet which can be economically manufactured in a large scale, has Br of 5.about.7 kG, and also has excellent magnetizing properties.
There are applications that demand higher B such as magnetic sensors, speakers, actuators, and stepping motors; and for these applications, the Sm.sub.2 C.sub.17 anisotropic resin bonded magnet is presently used as the highest performing magnet, and the Nd--Fe--B isotropic resin bonded magnet as a lower cost replacement magnet. But, these magnets are still costly, and it is desired to have a low cost, easy to manufacture resin bonded magnetic material possessing high Br characteristic.
On the other hand, in the Nd--Fe--B system magnet, magnet material in which Fe.sub.3 B type compound is the predominant phase in the vicinity of Nd.sub.4 Fe.sub.77 B.sub.19 (at %), was recently proposed, (R. Coehoorn et al., J. de Phy. C8, 1988, pages 669.about.670). This magnet material is obtained by a heat treatment of amorphous ribbons, resulting in the metastable structure which contains the crystalline cluster structure of Fe.sub.3 B and Nd.sub.2 Fe.sub.14 B. Br of the metastable structure reaches even to 13 kOe, but its iHc of 2.about.3 kOe is not sufficiently high enough. Also, the heat treatment condition are very limited, and it is not practical for the industrial production.
Studies have been reported in which additive elements are introduced to magnet material to make it multicomponent and to improve its magnetic characteristic. One of them utilizes Dy and Tb in addition to the rare earth element, Nd, to attempt to improve iHc; however, the problem is the high cost of additive elements, and reduced magnetization due to the fact that magnetic moments of rare earth elements couple anti-parallel to magnetic moments of Nd and Fe, (R. Coehoon, J. Magn. Magn. Mat, 89 (1991) pages 228.about.230)
The other study (Shen Bao-gen, etal, J Magn. Magn. Mat, 89(1991) Pages 335.about.340) replaces a part of Fe by Co to increase curie temperature to improve the temperature coefficient of iHc, but it has the problem of reducing B with addition of Co.
In any case, the Fe.sub.3 B type Nd--Fe--B system magnet is made amorphous by the melt-quenching method using a revolving roll, and heat treating it to obtain the hard magnet material. However, the resultant iHc is low, and the heat treatment condition mentioned earlier is very severe; and the attempt to increase iHc resulted, for example, in lowering the magnetic energy product, and the reliable industrial production is not feasible. Therefore, it cannot economically replace the ferrite magnet as its substitute.
This invention, focusing on the Fe.sub.3 B type Fe--B--R system magnet (R=rare earth elements), by increasing iHc and (BH)max, intends to establish the manufacturing method which enables the reliable industrial production, and provide a Fe.sub.3 B type Fe--B--R system resin bonded magnet with more than 5 kG of the residual magnetic flux density (Br) as an economical substitute for hard ferrite magnets.
Also, in order to provide the reliable and inexpensive Fe.sub.3 B type Fe--B--R resin bonded magnet with more than 5 kG of the residual magnetic density (Br), this invention intends to provide the most suitable rare earth magnet alloy powder for resin bonded magnets and their production method.